(1) Field of the Invention
The present invention relates to level-shift circuits and, more particularly, to a level-shift circuit in an output stage of a high-withstanding driver for driving such as a flat-panel display and an electrostatic plotter.
(2) Description of the Related Art
Conventional level-shift circuits to which the present invention relates and which are frequently employed for output stages of high-withstanding drivers as noted above are shown in FIGS. 1A, 1B and FIG. 2. These circuits will now be described.
The conventional circuit shown in FIG. 1A is a level-shift circuit of BiCMOS type (Bipolar-CMOS) which uses a bipolar transistor Q.sub.P and MOS field-effect transistors (hereinafter referred to as "MOS transistor"s). The output PNP-transistor Q.sub.P is controlled by on-off controlling of its base current.
The conventional circuit shown in FIG. 1B is formed by MOS transistors alone. The gate-source voltage of the output stage P-channel MOS transistor P4 is controlled by on-off controlling of a bias current flowing through a resistor R.
The conventional circuit shown in FIG. 2 is an example of a flip-flop type level-shift circuit. This circuit features that no steady-state (or constant) bias current is necessary.
The level-shift circuits as described above are employed for output stages of various high-withstanding drivers. The uses of the high-withstanding driver are classified into those requiring comparatively large output currents as used in flat-panel displays and those without need of any large output current as used in electrostatic plotters.
In the case where a small current suffices for the purpose, it is necessary to protect the circuit against the destruction caused by, for example, short-circuiting of an output, by rather limiting the output current with such means as a current limiter.
To this end, a protection resistor having a high resistance value is connected to the output terminal. This resistor usually has a resistance value of 100K.OMEGA. or above.
However, the provision of such protection resistor which is externally connected to the driver's output terminal necessitates extra steps for control and connection of parts and materials, thus increasing the cost of manufacture. In additon, an increase of the number of connections is undesirable from the standpoint of the reliability of connections. Such problems posed by the external connection of the protection resistor are particularly serious for drivers which have a large number of output terminals.
It may be thought to provide the protection resistor as above inside the driver. However, a resistor having a resistance value of 100K.OMEGA. or above requires a very large occupying area if it is to be realized on a semiconductor substrate, and the provision of a large number of resistors occupying such a large area is very disadvantageous for the integration density increase and the size reduction of the driver.
It may be further thought to form the output stage of the driver as a constant-current output circuit. With the conventional circuits, however, it is difficult to provide a constant-current output as a high level output although it is easy to provide a constant-current output as the low level output.